1. Field of the Invention
The present invention relates to radiation spectroscopy generally and, more particularly, but not by way of limitation, to a novel stabilized scintillation detector and a method of use thereof.
2. Background Art
In U.S. Pat. No. 4,160,165, McCombs and Stein describe a gain stabilization circuit applicable for scintillation detectors working in current mode. The stabilization is achieved by using light emitting diode (LED) that simultaneously illuminates two photodetectors. The first photo detector is a photo-multiplier tube (PMT) that is a part of the scintillation detector. That is, the PMT photocathode is illuminated by light from both the scintillation crystal and the LED. The second photo detector is a solid state photodiode that is illuminated only by the LED light.
The scintillation crystal is exposed to radiation coming from al x-ray beam. The LED is connected in the feedback loop of ether the PMT signal chain or the PD signal chain. By switching the feedback loop at two different radiation intensities, the difference between these two intensities can be measured independently of the PMT gain characteristics. This circuit arrangement performs quite well for this particular application—to measure the difference between two radiation intensities.
Shortcomings of the circuit of the '165 patent are:                The PMT must operate in current mode—spectroscopy devices require operation in pulse mode;        Applicable only for measurement of the difference between two non-zero radiation intensities (relative measurement)—spectroscopy measurements are absolute and do not have control of the radiation intensity, there is no beam on-off signal; and        The LED must emit light continuously—this is a noise source when operating in pulse mode.        
Radiation spectroscopy using scintillation detectors requires that the photomultiplier tube operate in pulse mode on an event-by-event basis. Each event produces a very small amount of light. Therefore, a common stabilization technique is to use an LED that is pulsed periodically. The LED is off between the flashes, ensuring low noise measurement of the radiation events.
U.S. Pat. Nos. 6,342,698; 5,859,429; 5,548,111; 5,237,173; 5,004,904; 4,605,856; and 4,160,165 describe stabilization techniques that use LEDs operating in pulse mode. The essential part is that the LED must produce a stable reference light that does not change with temperature and in time. This requirement is difficult to achieve and may require extensive calibration procedures. All these techniques are aimed to stabilize the gain of the photo-multiplier. However, the overall stability depends also on the temperature stability of the scintillation crystal which has to be taken into account.
Accordingly, it is a principal object of the present invention to provide a technique and circuit arrangement to stabilize the scintillation probe.
A further object of the invention is to provide such a stabilized scintillation detector and method that eliminate PMT gain instabilities and account for scintillation crystal temperature variations.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.